1. Field of the Invention
The present invention pertains generally to inclinometers and more specifically to high resolution digital inclinometers.
2. Background of the Invention
This application comprises an improvement of a previously filed and co-pending application Ser. No. 737,786 entitled "HIGH RESOLUTION DIGITAL INCLINOMETER" filed May 28, 1985 by Floyd J. Mills, and now Pat. No. 4,606,133, which is specifically incorporated herein by reference for all that it discloses.
The previously filed application disclosed a portable hand-held inclinometer which has high resolution and is capable of providing a digital readout of inclination relative to various angles such as level, plumb and preselected tare angles. It also disclosed a device which is capable of measuring difference angles between two surfaces and the ability to produce an audible tone when the device is inclined at an preselected angle, the ability to hold a specific angle for display and other automated features coupled with a high resolution instrument which has not been shown in the prior art and which incorporates features which are greatly needed in such a device.
A patentability search was performed prior to filing the previously filed application. The following references were uncovered in the search.
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 2,407,580 Scott Sept. 10, 1946 2,598,355 Cloud May 27, 1952 2,924,022 Callahan Feb. 9, 1960 2,952,920 Cloud Sept. 20, 1960 3,950,859 Kramer April 20, 1976 4,096,638 Schimming June 27, 1978 4,277,895 Wiklund July 14, 1981 4,486,844 Brunson et al. Dec. 4, 1984 ______________________________________
A discussion of these references, as it appears in the previously filed application, is set forth below:
U.S. Pat. No. 2,407,580 issued to Scott on Sept. 10, 1946 discloses a long period pendulum which utilizes a light source 18 which is focused by a lense 20 on a Lucite fly wheel 6. Photo cell 26 is arranged to detect light which is transmitted through the Lucite wheel 6. The Lucite wheel is mounted on a ribbon 4 and disposed on a housing which is filled with a liquid 28 having about the same specific gravity as the Lucite fly wheel. A portion of the Lucite fly wheel is painted with an opaque paint to block light as it flows through the wheel. A portion of the wheel is slightly weighted so that it tends to remain in a given rotational position on its axis. A ribbon support of the fly wheel gives practically no static friction and very small restoring forces are produced as compared to the gravitational force on the unbalanced fly wheel. A liquid of approximately the same specific gravity as the fly wheel unweights the ribbon and provides the proper amount of friction to obtain critical damping. The amount of light transmitted through the wheel determines the inclination of the device which is used to drive a servo to return a platform to a level position.
U.S. Pat. No. 2,924,022 issued to Callahan on Feb. 9, 1960 discloses rotary indicators utilizing a pendulum 23 which is placed in equilibrium by the use of a buoyancy float 59 to provide buoyancy approximately equal to the weight of the moving assembly. This is used to minimize and substantially eliminate friction on bearings 17 and 18 which function primarily as guides once the device is in equilibrium. As illustrated in FIG. 10, the rotary indicator comprises a mechanical pointer which aligns with a dial on the device to indicate the rotary position of the inclinometer.
U.S. Pat. No. 3,950,859 issued to Kramer on Apr. 20, 1976 discloses an angular displacement measuring apparatus which has electronic circuitry for determining instantaneous angular displacement relative to an external magnetic field or angular displacement in a verticl direction. The Kramer device uses a disk having sequences of transparent and opaque cells in circular tracks which are arranged relative to light sources and track oriented photo-sensitive devices to provide data relative to the orientation of the disk. FIG. 3 discloses standard bearing support without the use of floatation for eliminating frictional forces.
U.S. Pat. No. 2,952,920 issued to Cloud on Sept. 20, 1960 discloses a ballast compensate pendulum which utilizes a ballast chamber 22 immersed in a damping fluid 20 to provide buoyant effects which are equally distributed around the axis of wire support 11 and approximate the weight of the disk. Portions 12 and 13 of the disk are removed to cause the device to act as a pendulum. Adjutment screw 29 adjusts the pressure on fluid 20 to precisely place disk 10 in equilibrium within fluid 20. The Cloud patent does not disclose a sensing device but refers to U.S. Pat. No. 2,598,355 issued to Cloud on May 27, 1952 which uses a photo-electric cell assembly to sense the unequal distribution of light which is amplified by an amplifier to energize a servomotor.
U.S. Pat. No. 4,096,638 issued to Schimming on June 27, 1978 discloses a pendulum device having floats 102 which partially eliminate friction on bearings 92 and have pointers 98 which indicate the inclination of the device. Floats 102 float on the surface of the fluid disposed in the device.
U.S. Pat. No. 4,277,895 issued to Wiklund on June 14, 1981 discloses an apparatus for measuring acceleration which uses magnetic forces. Light emitting diodes 9 and 10 are located on one side of the plate 8 and a corresponding pair of light detectors 12 and 13 are located on the other side of the plate. The current required to produce a magnetic field to maintain the position of plate 18 is directly proportional to the acceleration of the device. A microprocessor 60 is utilized to quickly and accurately generate the required correction current.
U.S. Pat. No. 4,486,844 issued to Brunson et al. on Dec. 4, 1984 discloses a dual axis inclination measuring apparatus and method which has two sensor devices for measuring the inclination of two surfaces 18 and 19. Indicator unit 3 is capable of freezing a particular reading on its display and can produce a difference angle reading indicative of a difference in inclination of the two surfaces 18 and 19.
As can be seen from these references, the general concept of the use of buoyancy to place the moveable portion of an inclinometer in equilibrium in a fluid to reduce frictional forces has generally been shown in the Scott, Callahan, Cloud and somewhat in the Schimming patents. Additionally, optical detection of the position of a disk has been shown in the Scott and Cloud patents. These devices are relatively crude devices for producing a difference signal by detecting the total amount of light transmitted through a partially opaque disk.
Kramer discloses a more elaborate system of detecting the position of a digitally encoded disk to more precisely determine the location of the disk. Although Kramer is capable of producing a digital display of the position of the disk, Kramer does not use buoyancy to substantially eliminate frictional effects and produce a high resolution output. Moreover, the hard wired electronics utilized by Kramer, as illutrated in FIG. 6, are merely capable of indicating the absolute position of the disk and cannot produce digital displays of inclination relative to the various angles such as plumb, level and tare. Kramer is incapable of performing program functions which can be carried out by microprocessor control to allow the flexibility of producing digital read-outs which can indicate these various angles, as well as providing a display of difference angles between two surfaces and generating an audible tone. The disk utilized by Kramer constitutes an absolute encoding disk which is used to produce a direct read-out of the angular position of the disk which is proportional to inclination. To produce a high resolution read-out with an absolute encoding disk, a large number of tracks would be required as well as a large number of detectors and associated circuitry for reading these tracks. Hence, Kramer cannot provide flexibility in producing a digital display signal and cannot practically provide a high resolution read-out.
The Wiklund device discloses the use of a microprocessor in conjunction with an accelerometer. However, the Wiklund device utilizes the microprocessor to calculate and control the generation of error currents for energizing electromagnetic coils to maintain the pendulum in a centered position. The microprocessor device of Wiklund is not used for performing various program functions to provide various digital displays of inclination.
The Brunson et al. reference, on the other hand, discloses the use of a computer to generate angle readings which constitute difference angles of the inclination between two remotely located surfaces. Although Brunson et al. discloses the generation of a difference angle, there is no disclosure of program functions for generating digital signals of inclination relative to plumb, level, and tare angles, the use of an audible indicator or the use of buoyancy to place a disk encoding wheel is equilibrium to provide a high resolution read-out. Rather, the Brunson et al. device is an extremely sensitive device for measuring very slight differences in the inclination of two surfaces 18 and 19 and is incapable of providing a high resolution read-out for a wide range of inclination angles and generation of difference angles from a single sensing unit. Moreover, Brunson et al. does not disclose or teach the use of buoyancy, in any manner, to produce an inclination angle signal.
During the course of prosecution of the previously filed application, the following references were cited by the Examiner.
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 2,990,622 Johnson 7-4-61 4,253,242 McInerney 3-3-81 4,467,527 North et al. 8-28-84 4,492,029 Tanaka et al. 1-8-85 Russian Patent 197,801 1-1978 Russian Patent 972,212 1-1982 ______________________________________
McInerney discloses a digital angle indicator having an enclosure 40 which is partially filled with a fluid. A series of slots 46, 48, 50 and 52 are aligned with a plurality of optical devices 54, 56 to provide alignment information. McInerney provides essentially 8 positional information data signals and a low resolution system. McInerney does not use equilibrium to eliminate frictional forces between shaft 32 and bearings 30A and 30B.
Johnson discloses a combination protractor and level which uses a disk having a weight index medium aligned with the weight in graduations on the dial for indicating inclination angles. The disk is supported by a shaft 38. A fluid 42 is used as a lubricant for the shaft and to provide shock absorption and stabilization of leveling wheel, as disclosed on lines 12+, column 3. Johnson does not disclose the use of buoyancy to obtain equilibrium and to eliminate frictional forces.
A possible disadvantage of the high resolution inclinometer as disclosed in the previously filed application is that large changes in temperature and/or pressure may cause voids to form in the fluid cavity. This is the result of differential expansion or contraction of the fluid as a result of these temperatures and/or pressure changes. These voids can form microbubbles which may interfere with the optical detection process used to provide the high resolution inclination angle. For example, the microbubble voids may refract light generated by the light emitting diode so that the optical signal is improperly detected. Consequently, it is extremely important to prevent the formation of these voids for proper operation of a high resolution digital inclinometer using optical and digital encoding device.
Additionally, fabrication of the device is difficult if the optics must be disposed within the fluid cavity. The optics must be precisely aligned to provide proper readings and the fluid cavity must be perfectly sealed to prevent leakage of fluid or air from or into the fluid cavity. The ability to satisfy these requirements in a device which can be readily manufactured and assembled is difficult to achieve.
It is also desirable to design the inclinometer device to minimize the effects of friction when the device is not disposed in a vertical orientation, but rather, at a transverse angle causing the shaft to ride against the bearings and reduce the resolution. Determining the design criteria which affect the resolution of the inclinometer at transverse angles would allow the inclinometer to be designed for maximum resolution.